The present invention concerns a method for guiding a paper or board web in a paper manufacturing machine, a board manufacturing machine, or finishing units in connection with such machines. The present invention also concerns an apparatus for using the wire guide rolls of the above-mentioned finishing units for guiding the web.
In the following, the present invention is described in connection with a paper manufacturing machine or paper finishing units but it is equally well applicable in connection with board manufacturing machinery.
In modern high-speed paper manufacturing machines and multi-station coaters, the web being treated or prepared must travel straight in an accurately aligned manner. As the length of the machines as well as the distance covered by the web increases with the increasing speeds of the machines, an increase in speed affects web control in many ways. At high speeds, controlling the web is made more difficult by an air film formed between the web and the rolls, the film preventing even an accurately aligned set of rolls in a machine from hindering lateral transfer of the web caused by external forces. External forces affecting web travel are caused by, e.g., an imbalance between the blast driers, uneven web tension, moisture, and coat weight profiles. Constructional features affecting web travel include the perpendicularity of the rolls as well as the level of the roll ends, the symmetry of the suction effect of the airborne blast driers and the distance thereof from the web, particularly differences in the distance between the front and the driving side of the machine, the perpendicularity of the unwinder and possibly the unequal sidedness of the web. Naturally, the aim is to construct a coater such that the web travels straight, and that, should the web travel be skew due to, e.g., defective installation, the problem can be corrected by adjusting the machine.
Web transfer that is not due to constructional features of the machine is more difficult to control. With the rising speed of a wire-furnished cylinder group, for example, a phenomenon occurs where an air film is formed between the smooth cylinder surface and the paper web whereby friction vanishes and, because of the effect of centrifugal forces, the web is pressed against the drying wire which thus alone determines the travel of the web. In a high-speed machine, a rough and air-permeable drying wire together with the unwinding/reeling section and the blade sections offers one of the few chances for the web to be in proper frictional contact with a surface.
Efforts have been made to improve the contact between the web and the rolls, among other solutions, by providing the rolls with air exhaust channels, thus attaining a longer-lasting guiding effect of the rolls. However, this will not suffice when high travelling speeds are applied, because the amount of air handled by the air exhaust channels will be considerable and the web will form an air film over the roll ridges in spite of the channels. At high speeds, centrifugal forces will detach the web from the rolls unless very high web tension levels are used. This, again, is impossible due to the risk of web breaks. In future high-speed coaters these points will be accentuated because a probable line of development comprises switching over to machines with an essentially airborne web. In airborne web travel even the smallest imbalance between the blast driers causes transfer of the web, and the balance between the blast driers is more difficult to control than the perpendicularity of the rolls.
The aim of the present invention is to achieve a method and apparatus for effective active guiding of the travel of the web in the lateral direction of the machine.
According to the present invention, the travel position of at least one wire travelling in contact with the web is altered such that the velocity vector of the wire at least at the point of contact of the web and the wire is directed sideways in relation to the travel direction of the center line of the web when the web is brought onto the wire.
The present invention makes use of the phenomenon observed in coaters whereby the position of the web can be most efficiently influenced. Advantageously, the position of the web is controlled by means of the wire of the drying cylinder group by adjusting the perpendicularity of the wire stretching roll. The method can be applied to existing machines without changing the machine construction in any way. Then the adjustment of the web position is carried out manually. The adjustment can even easily be carried out automatically, whereby, e.g., the wire stretching roll is altered such that it can be adjusted by using a screw jack or a similar actuator unit. By altering the position of the wire stretching roll by means of an actuator unit the travel of the web can be actively influenced and the web can be made to stay centrally positioned in the machine even with changing travelling conditions. Adjustment of the web position during travel is most advantageously carried out in connection with web position gauging.
By means of the wire, effective web guiding is achieved even in high-speed machinery because the web is easily pressed against an air-permeable web. This makes it possible to guide the wire even in machines where the web is mostly guided in an airborne manner. There is no need to increase web tension, nor is the risk of web breaks increased. During test runs wire strength was not impaired, and the wire guiding effect on the position of the web was clear and significant.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.